1. Field of the Invention
The present invention relates to a combined engine for a single-stage spacecraft that employs oxygen in the atmosphere as oxidizer whilst within the aerospace and which is capable of achieving acceleration of the speed of flight from take-off speeds to orbital speeds with a single engine.
2. Description of the Related Art
The space shuttle is a known partially recoverable system for traveling back and forth between the ground and space orbits; however, the space shuttle carries a lot of propellants such as liquid fuel and oxidizer. The propellants occupy almost more than 80 percent of the gross weight at lift-off. There is therefore a limit to the extent to which the payload ratio with respect to the gross lift-off weight of the space shuttle can be increased. A recoverable spacecraft must rely on a rocket engine for traveling in space and at high altitudes where the air is very thin. However, consideration has been given to a combined engine of the air-breathing type, in which the amount of oxidizer carried can be reduced by utilizing the oxygen in the atmosphere as oxidizer when ascending through the atmosphere. Further, the engine generates higher performance than the pure rocket engine, as far as the vehicle can capture enough mass flow rate of air for its air-breathing propulsion engine.
The main thrust of current research and development has concerned the development of a two-stage spacecraft. There have been studied many concepts. The air turbo ramjet/rocket engine is one of them, in which the like utilizing atmospheric oxygen as the oxidizer is employed as the first stage and a rocket engine is employed as the second stage after the first-stage engine has separated. However, in the case of a two-stage spacecraft, tasks such as the development/manufacture of engines of two different types, coupling and separation of the rocket engine with the air turbo ramjet and maintenance and repair etc of the air turbo ramjet are necessary, resulting in higher operating costs. Although safety could be improved compared with that of the space shuttle, it cannot be expected that operating costs can be greatly lowered below those of pure rocket powered launch systems including the current space shuttle.
Instead of a two-stage spacecraft, a single-stage spacecraft may be advocated. In which flight is achieved with multiple propulsion systems and in which no mutual separation takes place from take-off from the runway up to Earth orbits (altitude about 50 kilometers or more, speed about 7000 meters per second or more). However, for such a spacecraft, the integrated engines having engine exhaust velocity of more than that of a chemical rocket engine (i.e. about 8000 meters per second or more) would be necessary and such an engine has not yet been developed for very high flight speeds beyond Mach No. 3.5.
A large number of conceptual studies and patent applications have been made concerning single-stage spacecraft. Practically all of these, however, concern engines wherein two or more engines of different types are mounted on the fuselage and the type of engine operating is changed over in accordance with the flying speed (see for example Japanese Laid-Open Patent Publication No. H7-34969) or an air-breathing Brayton cycle engine (see for example Japanese Laid-Open Patent Publication No. H7-4314) having a construction of variable shape, in which the shape is altered depending on the flying speed.
Combined engines, in which two or more engines of different type are mounted on the fuselage, have been studied for long time. For example, there are a LACE (liquid air cycle engine) capable of being allocated the speed region from take-off to about Mach 5 and the speed region of Mach 20 and above, and a scramjet engine capable of being allocated the speed region from the vicinity of Mach 4 to the vicinity of Mach 25. However, if the scramjet is supposed to be that of the Brayton-cycle, there has been never studied any of such engines effective beyond Mach number 8. Further, in such combined engines, air intake port changeover is required in order to change the air intake to the allocated engine in accordance with flight conditions. In ultra-supersonic flight, the problem arises of high thermal loading due to conversion of the increasing dynamic pressure of the admitted air into higher heat. However, regarding the moveable device that is required to effect changeover of the air intake port, there are technical problems in the design of a heat-resistant structure including heat-resistant materials capable of coping with such high thermal loads, which make such a device difficult to implement at the present time. As the range of flight speed is made wider, it becomes necessary to adopt expedients such as making the shape of the passage cross-section of the air intake section or the shape of the nozzle variable. Such a construction in which the air intake section or nozzle shape is made variable requires the provision of a large number of actuators for making the shape variable and is subject to problems such as that the construction of the engine becomes complicated and manufacturing costs are increased.
A geometry variable Brayton ram/scram-combined-jet engine provides with for example a flow path linking a front aperture section, a central hollow section and an aft aperture section, and all these sections are required variable geometry to change the flow path area. Further, it is required to provide with a device for injecting hydrogen into the central hollow section, and part of the member constituting the periphery of the flow path being of a moveable construction and additionally provided with a device for closing the front aperture. During atmospheric flight, it functions as a jet engine, burning the hydrogen using atmospheric oxygen; during flight outside the atmosphere, the air intake port is closed and another rocket engine system is required onboard to inject the vehicle into the mission space orbit. Because the ram/scram-combined-jet engine starts only from flight Mach number of about 5, the vehicle has to have another propulsion engine system to arrive the flight speed from the ground take-off. This combined engine also is difficult to implement, for the same reasons as in the case of a spacecraft carrying engines of two or more types. Additionally, its high-speed operating limit is predicted to be actually about 10 to 12 in terms of flight Mach number, due to limitations regarding the supersonic diffuser that is required for supersonic combustion (lowering the incoming air local Mach number prior to combustion, raising the static pressure and ensuring the necessary combustion chamber pressure), and because the released energy of the combustion becomes much smaller than the total energy of the incoming air, i.e. the energy required to accelerate the incoming air to the atmosphere is lost due to the internal momentum losses.
Thus, with a single-stage spacecraft, in regard to altering the shape of the wall surfaces of the air intake section or supersonic variable diffuser in response to flight speed, there are many aspects whose solution is difficult from the technical and cost point of view. If a sophisticated fluid dynamics is applied to the engine mainstream, an air intake section and supersonic diffuser whose wall surface shape will not be needed to change. Whereas, in the conventional Brayton cycle engine, this was sought be achieved by an air intake section and supersonic variable diffuser of variable shape during supersonic/ultra-supersonic flight, produced by mechanically changing the shape of the wall surface. An innovative method of making the intake of air to the combustion chamber variable in accordance with the flight speed by a more simple and inexpensive configuration is therefore sought. Accordingly, there exists the problem to be solved, in a wide range of flight speeds, to make possible changeover of engine mode by a fluid dynamic concept without variable geometry configuration.